I think Lee was a man caught between two fires. He was loyal to his oath as a soldier and made several statements that would lead you to believe that he would remain loyal to the Union. I think that was what lead to his being offered the appointment as commander of the Union Army. That and his almost being the highest ranking officer of the last of the senior command staff that had not resigned his commission and joined the southern army at the time. All those who had held higher rank had resigned and accepted commissions in the Confederate army, Samuel Cooper, Albert Sidney Johnston, Joe Johnson, P.T.G. Beauregard, all held rank high than Lee in the regular U.S. Army prior to their resignations. These same men were held high rank than Lee in the Confederate Army also.

But something changed in Lee's attitude when Fort Sumter was fired on and Virginia seceeded and I don't believe that it was just simply loyality to his state.

Lee had to have been privy to at least some of these plans being so closely associated with Winfield Scott as he was. I think that once Lee realized that Lincoln and his cohorts really intended to wage war against the Southern States, especially Virginia and his home, and that these acts by Lincoln, which up to that time could have been construed to be honest efforts to preserve the Union, were in fact going to lead to a war when Lincoln called for the 75,000 volunteers to put down the rebellion, and that there was no other solution other than war, at that point I believe that Lee took the path that preserved the best of the original intent of a constitutional government and resigned.

As a man of Honor Lee was still, in effect, holding true to his original oath to the uphold the Constitution.